This invention relates to compressors and blowers, especially those intended for supplying generous quantities of air at moderate pressures such as one to two atmospheres above ambient.
The invention is particularly related to an improvement in multiple stage centrifugal compressors.
Centrifugal compressors are well known and have been employed in a variety of applications. For example, centrifugal compressors are described in U.S. Pat. No. 4,646,530; U.S. Pat. No. 4,262,988; U.S. Pat. No. 2,888,809; and U.S. Pat. No. 3,362,625. A multiple stage centrifugal compressor is described in U.S. Pat. No. 4,429,540. Another multiple stage centrifugal compressor is described in U.S. Pat. No. 3,976,395.
In any typical centrifugal compressor, gas is introduced to a rotary impeller which drives the gas outward at high velocity through a radial compression channel into an annular diffusion chamber. In this chamber, the velocity of the gas drops and its pressure increases. That is, the velocity (kinetic energy of the gas) is converted into pressure (potential energy). In a single-stage unit, the compressed gas can be drawn off from the diffusion chamber. However, in a multiple stage compressor, the compressed gas continues from the diffusion chamber into a radial return channel, where the gas is led radially inward to feed the next stage. An inlet passage turns this flow of return compressed gas between 90 degrees and 180 degrees to introduce a flow of compressed gas to the impeller of the next stage, where the process is repeated.
At the inlet passage, the gas turns around a small radius on the radially outer, or shroud side, and around a large radius at the radially inner, or hub side. The small radius of curvature of the gas passage at the shroud side for the relatively wide passage area (due to the much larger radius of curvature on the hub side) leads to flow separation. At the high velocities experienced in compressor operation, this flow separation results in substantial performance degradation, because of pressure loss and efficiency reduction.
In existing multiple stage blowers of this type, a single baffle ring is installed in the inlet passage, positioned somewhat closer to the shroud than to the hub. The exact location of the ring has not been regarded as critical. The object of the baffle ring has been to prevent flow separation where the moving air flow has to make a sharp 180 degree bend from the return channel to the impeller of the next stage.
Testing of the conventional baffle ring configuration has revealed a measurable improvement of efficiency. A single baffle ring installed somewhat closer to the shroud than to the hub has been found to increase the overall blower efficiency by about eight percent over the same unit without the baffle ring. However, additional baffle rings did not improve the efficiency. It was tried to produce higher efficiency by installing a second baffle ring between the first baffle ring and the hub contour, thus roughly equalizing the spacings for the three resulting flow subchannels. However, this configuration caused a reduction in performance by two percent compared with the single baffle ring unit.
In other words, increasing the blower efficiency and performance was not simply a matter of installing baffle rings, because it was not previously appreciated how significant were the spacings of the baffle rings and the dimensions of the resulting flow subchannels.